In the previous article , I have explained the movement of a four legged robot in forward motion as well as how it takes turns. In this article I will explain the advantages of a four legged robot over a wheeled robot, support polygon, stability margin and sensors.
Before moving further, let’s first examine how these robots perform on an inclined surface. Both will move forward with stability. What will happen If the angle of inclination is increased a bit, the wheeled robot will topple, while the legged robot will remain stable as shown in fig 1. Do you know why this happened? To get the answer this question, let me explain the concept of a support polygon in a robot.
I have drawn a polygon in fig 2 using points of contact with the ground. This polygon is known as a support polygon. If the self-weight projection of the robot is within the support polygon, it means the robot is stable. Otherwise, the robot becomes unstable (refer fig 2). When I increase the surface slope, the projection of the centre of gravity begins to move backside, and after a certain angle, it lies outside the polygon. As a result, the robot becomes highly unstable and the robot loses its support and collapses.
Now, let me explain how this high inclination problem is solved using a four-legged robot.
The support polygon of the legged robot is formed by the legs in contact with the ground. An interesting thing here is that the support polygon becomes a triangle when the leg is lifted. What will happen if I slowly increase the surface slope?. When the projection point is outside the support polygon, the robot also becomes unstable. However, this robot can tilt its body by increasing the length of its back legs (refer fig 3). Due to this, the projection point falls inside the support polygon again. This kind of operation is not possible with a wheeled robot. This robot is stable now.
However, we can improve stability even further by understanding a concept called stability margin. Consider these two cases (refer fig 4), they are the same robots but with different leg movements. The stability margin is defined with respect to the direction of movement. Just by observing the below image, you can easily understand what a stability margin is. A higher stability margin is always preferred since it is more likely to remain stable if an unexpected force or high momentum is applied.
In the course of forward motion such as on the moon, the robot takes help from many sensors. HD multispectral camera and LIDAR are the primary navigation sensors used to scan the environment (refer fig 5a). Apart from collecting data from these sensors, the robot also needs to detect its leg disturbances accurately to react in time and avoid tripping. The sensors used by the robot for locomotion are the inertial measurement unit, touch sensors on feet, encoders, and torque sensors (refer fig 5b). The touch sensors can tell whether the robot is in contact with the ground or not. The amount of force applied by the ground on the sensors can be estimated using the joint torque sensors. The position of each joint can be given by the encoders. These sensors work together to move the robot forward.
Although this robot is useful on challenging terrain, it has some disadvantages like the complex design and control system, as well as the slow walking speed, which makes it highly inefficient on flat surfaces.
Thanks for reading!